The present invention relates to an electroconductive silicone rubber composition or, more particularly, to an electroconductive silicone rubber composition having excellent moldability and curing behavior and capable of giving an electroconductive cured silicone rubber article having a relatively low hardness and still excellent rubbery properties and high electroconductivity. The invention relates also to an electroconductive cured silicone rubber article obtained by curing the composition.
It is widely known in the prior art that an electroconductive rubber composition can be obtained by compounding an insulating rubber composition with a substantial volume of a finely divided electroconductive material such as metal powders and carbon blacks and various types of electroconductive rubber compositions are under practical use. For example, a volume resistivity of 10.sup.5 to 10.sup.1 ohm.cm can be easily obtained by compounding an insulating rubber composition with carbon black.
Silicone rubbers as a class of insulating rubber compositions having excellent heat and cold resistance and weatherability can also be imparted with electroconductivity by compounding with a finely divided electroconductive material. Several grades of electroconductive silicone rubber compositions are available on the market and widely used in practice.
The finely divided electroconductive materials conventionally compounded in an electroconductive silicone rubber composition include carbonaceous powders such as carbon blacks and graphite powders, powders of a metal such as silver, nickel, copper and the like, carbon fibers, metal fibers and non-electroconductive particles and fibers plated with a metal on the surface. Silicone rubber compositions compounded with these electroconductive materials can be imparted with a decreased volume resistivity of 10.sup.10 to 10.sup.-3 ohm.cm depending on the type of the electroconductive material and the amount of loading thereof with substantially no adverse influences on the unique properties inherent in silicone rubbers. A highly electroconductive silicone rubber having a volume resistivity of 10.sup.5 ohm.cm or below can be obtained by using a large volume of carbon black or a powder of a metal such as silver, nickel and the like.
When a highly electroconductive silicone rubber having a volume resistivity of 5 ohm.cm or below is desired by using a carbon black such as acetylene black, the amount of the carbon black compounded in the silicone rubber composition must be increased so greatly that the silicone rubber composition is poorly workable in roll milling and molding and the cured silicone rubber articles thereof may be poor in the rubbery elasticity or have an excessively high hardness to impart brittleness to the rubber. Moreover, it is known that carbon blacks have an adverse influence on the curing behavior of silicone rubbers due to the surface properties of the carbon particles and impurities contained therein. This is the reason for the difficulties in obtaining a highly electroconductive silicone rubber having excellent mechanical properties by using a carbon black as the conductivity-imparting particulate material.
Japanese Patent Kokai 61-108661 discloses an electroconductive silicone rubber composition containing carbon black as the conductivity-imparting material and compounded with a powder obtained by crushing and pulverizing a cured silicone rubber. This electroconductive silicone rubber composition compounded with a powder of cured silicone rubber is indeed advantageous in obtaining a moderate hardness of the cured silicone rubber body even by increasing the amount of compounded carbon black and reproducibility of the electric conductivity of the cured silicone rubber articles thereof. The improvement obtained by this means, however, is still insufficient and no solution is given thereby of the problem of the poor workability and curing behavior of electroconductive silicone rubber composition.